Sheath for tubular storage battery electrodes and method for their production

ABSTRACT

A sheath for tubular storage battery electrodes has an inner layer of an inert fibrous material and an outer layer of thermoplastic material in the form of a net, perforated foil, or the like. The two layers are mechanically joined by heating the fibrous layer to a temperature sufficient to soften the adjacent surface of the thermoplastic material and cause a portion of the fibrous layer to be embedded in the surface of the thermoplastic layer.

This is a division of application Ser. No. 533,499, filed Dec. 17, 1974now U.S. Pat. No. 3,972,728.

BACKGROUND OF THE INVENTION

The present invention concerns sheaths for tubular electrodes used inelectrical accumulators or storage batteries. Such electrodes are usedin alkaline, as well as in lead storage batteries and comprise a numberof connected, electricity-conducting rod-like or wire-like bars whichare surrounded by active material. The latter, in turn is surrounded byan electrochemically inactive sheath.

The most important application of tubular electrodes is in lead storagebatteries but the invention is obviously not limited to these only.

The first sheaths for tubular electrodes consisted of tubes of hardrubber into which had been notched a large number of grooves or slitsparallel to one another and at right angles to the tube's axis. Sincetheir introduction, these tubes have been improved in various ways, andconsiderable progress has been made in the development of a tubeconsisting of two layers. These first consisted of an inner layer ofbraided glass fiber and a outer layer of a perforated plastic sheet orfilm. Several other types of sheaths were subsequently developed; amongthem types wih several tubes connected with one another. Also,individual sheaths consisting chiefly of braided glass fiber, which areimpregnated in order of one layer have been developed, such as, forexample, sheaths to obtain greater stiffness and protection againstchemical action. Although this improved the strength of braided glassfibers, it resulted in decreasing the porosity of the sheath, therebylowering its permeability to electrolyte.

Double-layered sheaths possess certain advantages, among them being arelatively simple manufacturing process. The great advantage from thestorage battery technology viewpoint, is that electrodes with goodmechanical qualities and a good length of service are obtained.

In the production of the electrodes one first casts a bar or rod of anelectricity-conducting material, for example, a lead-base alloy. In thiscase the grid consists of a molding or border with a number of barsprojecting from it and parallel to one another. Over these bars, thetubular sheaths are then drawn, whereupon the space between the sheathand the bars is filled with active material in the form of a powder. Itis impossible to carry out this filling operation if the two materialsof which the sheath is made are not tightly connected. The drawing ofthe tubes over the bars is also rendered difficult by a poor union ofthe two materials. It is, therefore, essential that the two differentmaterials in a tubular sheath consisting of two layers should besecurely connected with each other. In this way one obtains to someextent an improvement of the envelope's mechanical qualities; but thisis particularly important in connection with the introduction of theactive material.

Formerly, union of the materials in the two layers of the sheath waseffected by gluing. This, however, resulted in several disadvantages.Depending on the type used, glue had to dry or harden while the twolayers were held together by means of mechanical devices, entailing anincrease in the manufacturing costs and unnecessarily large equipmentfor the production of the sheaths. As the periods required for hardeningof the glue usually are relatively very long, solution-base glue hasbeen used for the most part. This is not desirable, however, with regardto the working environment. The gluing process also makes it difficultto obtain a sufficiently good union of the two material layers withoutthe addition of a considerable surplus of glue. Such a surplus is agreat disadvantage, as it impairs the qualities of the electrodes whichare produced with the sheaths. A surplus of glue may also bring aboutincreased production costs and environmental problems. A disadvantage ofgreater importance in connection with gluing is that it may provedifficult to effect a satisfactory union between the two layers. This,of course, depends on which material is used, and the conditions can beinfluenced by a certain preliminary treatment of the material.

OBJECTS AND SUMMARY OF THE INVENTION

One object of the present invention is to provide a novel,double-layered sheath for a tubular electrode.

Another object is to provide a double-layered sheath having improvedstrength and porosity.

A further object is to provide such a double-layered sheath in which thetwo layers are joined without the use of glue or any other extraneousadhesive.

Still a further object is to provide a novel method for making adouble-layered sheath for tubular storage battery electrodes.

Yet another object is to provide such a method whereby the two layersare mechanically joined without the use of a glue or other extraneousadhesive.

The foregoing objects and others which will become apparent to thoseskilled in the art are achieved in accordance with the present inventionin the manner described below.

In general, tubular electrode sheaths according to the present inventioncomprise two layers, namely an inner layer of fibrous material and anouter layer of thermoplastic material covering the inner material. Thetwo materials are so united that only a thin layer of the surface of thefibrous material is pressed into the surface of the thermoplasticmaterial facing it. In this way one obtains a mechanical union betweenthe two materials. For various reasons, it is not desirable to have thefibrous material penetrate more deeply into the thermoplastic materialthan is necessary for a satisfactory joining. The intention thus is notto produce a reinforcement of the thermoplastic material. Of course, thedifferent layers may comprise a number of different materials, butexperience has shown that it is best to use fibrous material consistingwholly or chiefly of glass fiber, and it is, therefore, preferred forthe present purpose. Glass fiber is a well-tried material foraccumulator electrodes, which has proved capable of giving goodqualities to the electrodes.

The thermoplastic material preferably is a polyolefine. Polyethylene andpolypropylene are especially preferred. In order to provide foreffective penetration of electrolyte, the thermoplastic outer layershould be readily pervious to aqueous electrolyte solutions.

In general, union of the fibrous and thermoplastic layers is achieved byheat-softening the thermoplastic surface adjacent the surface of thefibrous inner layer and pressing the two surfaces together. Heating canbe done directly or indirectly, in any of several ways, as will bedescribed in greater detail below.

Furthermore, both the inner fibrous layer and the outer thermoplasticlayer may take various forms. These also are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood, reference ismade to the detailed description which follows and to the accompanyingdrawings in which:

FIG. 1 is an elevation view of part of one embodiment of a tubularelectrode according to the present invention, enlarged to show detail,and with parts broken away to show the internal construction;

FIG. 2 is a section taken on line 2--2 of FIG. 1, also enlarged to showdetail, with the conductor rod and powdered active material omitted toenhance clarity;

FIG. 3 shows a preferred embodiment having a net with threads ofdifferent diameters running in two directions, A and B;

FIG. 4 shows a section through the net, at right angles to the threadsrunning in direction A;

FIG. 5 shows a section at right angles to the threads running indirection B;

FIG. 6 shows a fragment of a sheath according to the embodiment of FIG.3;

FIG. 7 is a cross-section on line 7--7 of FIG. 6; and

FIGS. 8 and 9 show another form of net in the same manner as FIGS. 4 and5.

DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the present invention, the fibrous material isprovided which comprises two layers. The inner of these layers consistsof a fibrous inert material. On the outside is found a thermo-plasticmaterial in the form of a net, perforated foil, or the like. The twolayers are connected with each other, at least mechanically. A preferredmaterial combination is that the fibrous material consists chiefly ofglass fibers and the thermo-plastic material is a polyolefine. Theadjacent surfaces of the materials are joined to each other by softeningthe thermo-plastic material. This may be done either directly throughheating of the plastic material or indirectly by first heating thefibrous material and then transferring the heat to the thermo-plasticmaterial, chiefly in the contact surfaces between the two materials.

In accordance with the present invention, the fibrous material is firstheated, whereupon the thermoplastic material is added. In this way theretakes place, at least in the points of contact between the two layers, asoftening or fusion of the thermoplastic material. Thus a union of thetwo materials is achieved. The heating can be done in various ways.Because of the nature of the sheath material it is suitable that thematerial is present on a mandrel of proper material, preferably of ametal such as steel. Proper heating methods are IR-irradiation andinduction heating. First the mandrel is heated, and the heat is thentransferred from it to the fibrous material. With inductive heating, themandrel is surrounded by a spool or coil, which is fed by an alternatingcurrent whose frequency can be very high in certain applications. Inthis way eddy-current losses in the mandrel are produced and a strongheating of the latter is obtained. It is also possible to obtain,through suitable frequency selection, dielectric heating of the fibrousmaterial, casued by inner molecular friction. The mandrel on which theheating takes place can then be so long that the forming of the tubularsheath takes place on it.

Referring to the drawings, FIG. 1 shows a unit of a tubular electrodeaccording to the present invention. As can be seen, the unit cancomprise the usual lead or lead alloy conducting rod 2, surrounded bypowdered active material 3. The two are encased in a two-layer tubularsheath, designated generally as 1, which comprises an inner fibrouslayer 4 and an outer foraminous layer of thermoplastic material 5. By"foraminous" is meant that the thermoplastic layer readily permits theeasy passage of electrolyte and generated gases. This is shownschematically by means of passageways 6 through the outer layer.

As already stated, an essential feature of this invention is the factthat the fibrous layer 4 is not completely embedded in the surroundingthermoplastic layer 5. Referring specifically to FIG. 2, it will be seenthat only a thin outer layer 8 of the fibrous material becomes attachedto the inner surface of the thermoplastic material.

The fibrous material can be made, in the familiar manner, of a braidedglass fiber tube, while the thermoplastic material is formed of a flatstrip or tape, or in the form of a net, as will be described in greaterdetail below. This tape can consist of perforated foil or some otheropen surface material. The tape is then given the desired geometric format the same time that it is fastened to the fibrous material. Thegeometric form does not need to have a circular cross-section; manyother forms are possible, such as, oval, rectangular, and the like. Thethermoplastic material can be distributed on at least two tapes, whichare joined together in a known manner simultaneously by means of thethermoplastic material. The joining together can then be carried out sothat the axially extending ribs 7 are formed on the outside of thesheath, as shown in FIG. 1.

In the constructions used hitherto it has been found suitable to employfibrous material in the form of braided hose or stocking. This meets therequirements with regard to handling, strength, and price. With regardto the production of the tubular sheaths according to the presentinvention, a braided material is also very suitable, as the fibrousmaterial is effectively enclosed or sealed off in the thermoplasticmaterial through fusion. This is important, as a braided material, ifpulled in its longitudinal direction, increases its length with acorresponding decrease of its diameter. This, of course, must be avoidedin the production and utilization of storage battery electrodes. Bymeans of the present invention, the braid is anchored through fusion inthe surface of the thermoplastic material.

As already mentioned, the thermoplastic material can be made to formaxially extending ribs or borders projecting from the exterior of thesheath, as shown in FIGS. 1 and 2. The function of these ribs is tomaintain distance to the nearest component, usually a separator. It istherefore suitable that each tubular sheath have at least two such ribsopposite each other on both sides of the tubular sheath. In order tofacilitate orientation of the tube in the assembled electrode and toavoid deformation, sheaths may be provided with a greater number of ribsthan the two shown in FIGS. 1 and 2.

According to an especially preferred aspect of the invention, thethermoplastic material is formed into a net of special construction.Such a net consists of threads which run in two different directions andmust be so oriented with respect to one another that a plane through thecenter axes of threads that run in one direction does not coincide witha plane through the center axes of the threads running in anotherdirection. By "plane" is meant the plane or curved surface which unitesthe center axes of the different threads, whether these threads have acircular or a different cross-section. By using nets of this type onecan obtain the result that only threads running in one direction arefused, wholly or to a large extent, into the surface of the fibrousmaterial and carry out the function of uniting the two layers, while thethreads running in the other direction are not united (not at all or toa lesser extent) with the fibrous material. Instead they function tomaintain a certain distance from the remaining components in the batterycells and of protecting the fibrous material from wear and tear. Variousforms of the invention and nets of thermoplastic material suited to themare shown in FIGS. 3-9.

Referring specifically to FIGS. 3-9, the net shown in FIG. 3 consists,by way of example, of polyethylene and has threads (9) running indirection A and other threads (10) running in direction B. Threads (9)and (10) can have the same or a different cross-section, and theirposition with regard to one another can vary, as shown in FIGS. 4 and 5,as well as in FIGS. 8 and 9.

According to a form of the invention preferred for lead storage batteryelectrodes, the fibrous layer consists of a braided stocking or hose ofglass fiber (11) and the surrounding tubular thermoplastic layer ofpolyethylene net, as shown schematically in FIGS. 6 and 7. By way ofillustration, the net may consist of threads with a diameter of at leastabout 0.5 mm, or a little more, and a width of mesh with a distancebetween the threads (1) of about 3.5-4 mm. The thread diameterspreferably are not the same. With such a net one obtains a free surfacebetween 50 and 70%, a free surface between 65 and 70% being preferable.In this way one obtains the best possible construction with respect tomechanical stability, ease of electrolyte diffusion, and removal of thegases formed in the electrode.

The sheath just described can be produced in the same manner as in thecase of the other embodiments described. It is, however, preferred toproduce a sheath in the following manner:

The thermoplastic material is extruded through a well-known heatedcircular or rectangular nozzle or die, so that one obtains directly anet of the desired appearance in the form of a tube. The fibrousmaterial is also fed through the nozzle simultaneously, and thereby isplaced on the inside of the thermoplastic material. In this way oneobtains a simple manufacturing process requiring only a few moments and,at the same time, the union of the two layers can be carried outdirectly after the nozzle, as the thermoplastic material continues to beheated. If necessary, extra heat is added. It may also be advisable toheat the fibrous material before the two layers are combined, which cantake place before the nozzle. By utilizing a different supply of heatthan the one which the plastic material gets in the nozzle, one alsoobtains this advantage: the transverse threads in the net, i.e., thosewhich in the finished sheath lie in a plane at right angles to the axisof the sheath, soften more rapidly than the longitudinal threads, as thetransverse threads can be made thinner and contain less material. Themajor part of the transverse threads can thus be pressed into thefibrous material, while the longitudinal threads continue to havesufficient strength to make possible a continuous feeding of the sheathmaterial and have little contact with the sheath fibers.

The present invention, in its various aspects, has been described andillustrated as being particularly applicable to use in lead-acid storagebatteries. Furthermore, the novel tubular electrode sheaths wereillustrated with glass fibers constituting the fibrous layer and apolyolefine (e.g., polyethylene or polypropylene) as the thermoplasticlayer. It will readily be appreciated by those skilled in the art thattubular electrode sheaths, as described, are equally applicable to theconstruction of alkaline batteries and that any fibrous material andthermoplastic resin which are inert to the constituents of theelectrolytes (acid or alkaline) can be substituted for the glass fibersand polyolefine, respectively. It will be further apparent that thepresent invention is not limited to the production of the sheaths by thespecific heating methods described; but that any procedural expedient bywhich adjacent surfaces of the fibrous and thermoplastic materials maybe thermally joined falls within the scope of the method according tothe present invention.

Other embodiments and variations thereof falling within the scope of thepresent invention will suggest themselves to those skilled in the artand it is not intended that the invention be limited except by theclaims which follow.

We claim:
 1. A method for producing a sheath for a tubular storagebattery electrode which comprises heating a layer of anelectro-chemically inert fibrous material, adding thereto a porousforaminous layer of thermoplastic material, whereby the heated fibrousmaterial causes at least local softening of the thermoplastic layer toenable at least surface fibers to become embedded in the softenedthermoplastic surface without materially reducing the porosity of theforaminous layer, and forming a tubular sheath from the combined doublelayer.
 2. A method according to claim 1 wherein the two layers areformed into a tube around a heated mandrel.
 3. A method according toclaim 1 wherein the doublelayered tube is heated by means of infra-redirradiation.
 4. A method according to claim 1 wherein the doublelayeredtube is heated inductively by means of high frequency radiation.
 5. Amethod according to claim 1 wherein the layer of thermoplastic materialis formed of at least one strip of perforated tape or foil coated withthe thermoplastic material and is formed into the desired geometric atthe time it is applied to the fibrous material.
 6. A method according toclaim 5 wherein at least two strips of tape or foil are joined to formaxially projecting external ribs in the seams between the strips.
 7. Amethod for producing a sheath for a tubular storage battery electrodeincluding a rod of conductive material surrounded by active materialwhich comprises:extruding a thermoplastic material through a heatednozzle for forming a tube-shaped net of said thermoplastic material;heating said nozzle to a temperature sufficient to cause the surfaces ofsaid tube-shaped net to soften while simultaneously passing anelectro-chemically inert fibrous material through the nozzle to form anannular fibrous layer on the inside of said extruded tube-shaped net;extruding said tube-shaped net with a pressure sufficient to cause onlythe outer fibers adjacent the inner surface of the tube-shaped net tobecome embedded in those surfaces of the net closest to the fibrouslayer to produce a double-layered tubular sheath in which the adjacentsurfaces of the inner fibrous layer and the outer thermoplastic layerare mechanically joined; placing the formed sheath in surroundingrelationship to the conductive rod; and filling the space between therod and the sheath with active material.